Aluminum and yttrium oxide coated thermal spray powder

ABSTRACT

A thermal spray composite is disclosed, comprised of a base constituent formed from at least one of the metals nickel, iron, cobalt and chromium, plus additional constituents comprising aluminum and yttrium oxide. Optionally, the base constituent additionally contains aluminum, and the additional constituents may further include molybdenum and/or cobalt. In a preferred form, the composite is a powder having an alloy core of the base constituent, the core having fine particles of the additional elements secured thereto with a binder. The process of thermal spraying the composite is also disclosed, and the resulting coatings have a high degree of high temperature corrosion resistance and tenacity compared to prior art thermal sprayed coatings.

This is a continuation-in-part of U.S. patent application Ser. No.597,139 filed Apr. 5, 1984, and now abandoned. This invention relates toa thermal spray powder which will produce metallic coatingscharacterized by tenacity and corrosion resistance, and to a process forthermal spraying such coatings.

BACKGROUND OF THE INVENTION

Thermal spraying, also known as flame spraying, involves the heatsoftening of a heat fusible material such as metal or ceramic, andpropelling the softened material in particulate form against a surfacewhich is to be coated. The heated particles strike the surface and bondthereto. A conventional thermal spray gun is used for the purpose ofboth heating and propelling the particles. In one type of thermal spraygun, the heat fusible material is supplied to the gun in powder form.Such powders are typically comprised of small particles, e.g., between100 mesh U.S. standard Screen size and about 5 microns.

A thermal spray gun normally utilizes a combustion or plasma flame toproduce the heat for melting of the powder particles. It is recognizedby those of skill in the art, however, that other heating means may beused as well, such as electric arcs, resistance heaters or inductionheaters, and these may be used alone or in combination with other formsof heaters. In a powder-type combustion thermal spray gun, the carriergas, which entrains and transports the powder, can be one of thecombustion gases or an inert gas such as nitrogen, or it can be simplycompressed air. In a plasma spray gun, the primary plasma gas isgenerally nitrogen or argon. Hydrogen or helium is usually added to theprimary gas. The carrier gas is generally the same as the primary plasmagas, although other gases, such as hydrocarbons, may be used in certainsituations.

The material alternatively may be fed into a heating zone in the form ofa rod or wire. In the wire type thermal spray gun, the rod or wire ofthe material to be sprayed is fed into the heating zone formed by aflame of some type, such as a combustion flame, where it is melted or atleast heat-softened and atomized, usually by blast gas, and thencepropelled in finely divided form onto the surface to be coated. In anarc wire gun two wires are melted in an electric arc struck between thewire ends, and the molten metal is atomized by compressed gas, usuallyair, and sprayed to a workpiece to be coated. The rod or wire may beconventionally formed as by drawing, or may be formed by sinteringtogether a powder, or by bonding together the powder by means of anorganic binder or other suitable binder which disintegrates in the heatof the heating zone, thereby releasing the powder to be sprayed infinely divided form. In other forms the wire may nave a coating sheathof one component and a core of the others, or may be made by twistingstrands of the components.

Coatings produced by thermal spaying alloys of nickel, iron, cobalt orcombinations thereof as a base metal which contain, in the alloy,chromium and optionally aluminum and/or other elements are used toprovide corrosion protection of metal components such as in gas turbineengines and boiler systems. Cobalt, for example, is used as either abase metal or an alloying alement to improve high temperature creep andstrength properties in cast and wrought super alloys. However, it iswell known that cobalt is not classified as an oxidation resistantmetal. Scaling and oxidation rates of unalloyed cobalt in air are manytimes those of nickel. The scaling and oxidation resistance ofcobalt-base alloys at high temperature is largely a function of chromiumcontent. As a result, cast or wrought parts fabricated of cobalt alloysfrequently require special coatings for protection.

However, in the thermal spraying of such nickel, iron or cobalt alloys,the bond strength, bend ductility and thickness limits of the resultantcoatings are often not satisfactory, even where the coatings are thermalsprayed using a plasma spray gun. Also, to obtain resistance to thecorrosive conditions in the application, an alloying element such asyttrium or a rear earth metal is often added, but thermal spray powdersor wires of such alloys are expensive to manufacture. Typical alloys ofthis type are disclosed, for example, in U.S. Pat. No. 4,313,760, and inU.K. Patent Application No. GB 2,028,378A published Mar. 5, 1980.

To achieve high density and improved bonding, alloy powders are plasmasprayed in a low pressure inert atmosphere chamber, an operation that isslow and costly and requires sophisticated vacuum and work handlingequipment. There are similar and even more complex problems with vapordeposition which is an alternative coating method known in the field.Chambers also preclude deposition onto large components such as boilers.

Coatings having improved bond strength may be thermal sprayed using acomposite powder formed of metals capable of reacting exothermically asdescribed in U.S. Pat. No. 3,322,515. One such powder has a nickel corewith about 5 percent by weight of fine particles of aluminum bonded tothe surface thereof with an organic binder. The core may be alloyed withanother metal such as chromium. Cobalt plus aluminum and molybdenum plusaluminum are others of many exothermic pairs mentioned therein.

Several other patents teach improved clad powders to produce thermalsprayed coatings having good bond strength and the capability of beingreadily machined. One is U.S. Pat. No. 3,841,901 which discloses apowder of nickel, copper or iron core coated with fine particles ofaluminum and molybdenum for producing a thermal sprayed coating whichhas good bond strength and can readily be machined. Similarly U.S. Pat.No. 4,181,525 teaches a thermal spray powder comprising particles havinga core of nickel, iron, copper, cobalt or alloys thereof coated with abinder containing discrete particles of aluminum and substantially purenickel, directed to coatings having improved machinability.

The composite powders disclosed in the above-mentioned patents aregenerally employed for bonding other coating materials to substratessuch as steel, or for producing single step coatings for machine elementapplications requiring wear resistance and finishing capability.However, there has been only limited success with composite powderswhere corrosion resistance is required. The reasons are not wellunderstood. In aqueous or moist environments, electrolytic problemsappear to be associated with the heterogeneous nature of the coatingsresulting from incomplete alloying of the cladding elements with thecore during the thermal spraying process. However, protection is alsolacking in dry, high temperature situations that are oxidizing or thatinvolve sulfates and chlorides in either oxidizing or reducingconditions. If coatings contain any free nickel, as results fromincomplete reaction or alloying during thermal sprayings thenickel-aluminum clad powder of U.S. Pat. No. 4,181,525, even where thepowder has a nickel chromium alloy core, the coatings are especiallyvulnerable to attack in certain corrosive conditions. The attack is notonly in the coating material but in the interface, weakening the bondand causing coatings to spall.

Chromium is used as an alloying element in a powder core to improvrcorrosion resistance of coatings of a thermal spray powder in which thecore is clad with aluminum. However, the chromium as a core or as anaddition has proven to reduce the bond strength of the thermal sprayedcoating. For example, aluminum clad chromium or nickel-chromium alloyprovides lower bond strength than aluminum clad nickel.

As taught in U.S. Pat. No. 3,322,515, for bonding purposes, iron is notof itself a satisfactory component in a composite with aluminum, andiron chromium alloy clad only with aluminum has especially poor bondingwhen thermal sprayed.

Thus, although composite thermal spray powders are known and availablewhich may produce satisfactory bond strength, high tenacity is desiredfor the corrosive environments, and the coatings produced from suchpowders are particularly lacking in sufficient corrosion resistane. Onthe other hand, the known alloy powders used for thermal sprayingcoatings for high temperature corrosion protection lack sufficient bondstrength.

U.S. Pat. No. 3,655,425 discloses a thermal spray powder having coreparticles of a metal coated with fine discrete particles of a ceramicsuch as to leave exposed a portion of the surface area of the metalcore. The discrete particles may additionally include another ceramic ora metal such as aluminum. In teaching the exposure of core surface, thepatent is particularly directed to the problem of carrying into thecoating ceramic particles that are difficult or impossible to melt inthe thermal spray process. A preferred example of ceramic in theabove-mentioned patent is boron. nitride to produce abradable coatings.Yttrium oxide is not mentioned therein as an example of a ceramic forcladding the core particles, nor is there any teaching or suggestionthat inclusion of any other oxide with aluminum in composite materialsmay enhance the bond strength and corrosion resistance of the thermalsprayed coatings.

U.S. Pat. No. 3,864,093 teaches a coated article, the coated layerhaving at least 2 percent by volume of metal oxide particles dispensedin a metal alloy matrix. Fourteen metal oxides plus spinel combinationsthereof are given for the meal oxide particles, yttrium oxide being oneof the 14 oxides listed. The metal alloy matrix may be an alloy of iron,cobalt or nickel with one or more of aluminum, silicon and chromium. Thecoatings are produced by plasma or detonation spraying of blends of theconstituent particles of metal oxide and metal alloy, and arecharacterized by a high hardness of at least 500 VHN (with 300 gramload) which is equivalent to Rc. 49. As taught in the patent, the metaloxide particles increase wear resistance of the coating withoutdegrading the normal oxidation resistance of the alloy matrix. There isno indication that the oxide inclusions may increase the oxidation andhot corrosion resistance or the bonding. In fact, it is generally knownin the art that thermal sprayed oxides bond poorly in comparison tometals.

Cored wire of iron sheath and nickel powder core for thermal sprayingwith an arc gun is discussed in a paper "Introduction of Cored Wires toArc Spraying" by H. Drzeniek, Wroclaw, H. D. Steffens and J. Beczkowiak,Dortmund presented at the 10th International Thermal Spraying Conferencein Essen, May 1983.

A composite wire taught in U.S. Pat. No. 4,276,353 is formed of a sheathof aluminum and a compacted powder core containing a major portion ofnickel and stainless steel and a minor portion of aluminum and metaloxide. The examples given for metal oxide are zirconium oxide and cobaltoxide. The object, according to the patent, is to produce a readilygrindable coating without loss of its other desired characteristics. Thepurpose of the coating is to provide a bearing surface for repairingmachinery parts. No guidance is provided therein for improved bondstrength or improved hot corrosion resistance.

In view of the foregoing, a primary object of the present invention isto provide a novel thermal spray material for producing tenaciousmetallic coatings characterized by both high bond strength and hotcorrosion resistance.

A further object of this invention is to provide an improved thermalspray process for producing tenacious metallic coating characterized byboth high bond strength and hot corrosion resistance.

BRIEF DESCRIPTION OF THE INVENTION

The foregoing and other objects of the present invention are achieved bya thermal spray composite according to the present invention whichcomprises, as a base constituent, at least one of the metals, nickel,iron, cobalt and chromium, plus additional constituents comprisingaluminum and yttrium oxide. Optionally, the base constituentadditionally contains aluminum, and the additional constituents mayfurther include molybdenum and/or cobalt.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, a metallic composite material hasbeen developed for thermal spraying onto metallic substrates byconventional thermal spray equipment. The coatings produced thereby arevery tenacious, having especially high bond strength and ductility. Thecoatings additionally have a high degree of high temperature corrosionresistance compared to prior art thermal sprayed coatings.

The thermal spray composite comprises a base constituent that may itselfbe a composite but is preferably an alloy of at least one of the metals,nickel, iron, cobalt and chromium. The base constituent may additionallycontain aluminum. The chromium, if present, is preferably in an amountof about 1 to 55 percent by weight of the alloy. The aluminum, ifpresent in the base constituent, should be in an amount of about 1 to 60percent by weight of the base constituent. In one desirable embodimentthe aluminum is in the base constituent in an amount of about 20 to 55percent, and the resulting coating formed from the composite of thepresent invention has the additional advantage of improved ductilityover the highly brittle coatings of flame sprayed alloy high in aluminumcontent. If chromium and aluminum are both present in the baseconstituent, preferably they should total about 1 to 55 percent byweight of the base constituent. Nickel-chromium alloy, iron-chromiumalloy and iron-aluminum alloy have each been found to be particularlyadvantageous as the base constituent, depending on the environmentalexposure to which the resulting coating is subjected.

In accordance with the invention, the additional constituents aluminum,yttrium oxide and, optionally, molybdenum and/or cobalt are eachadditionally present in the thermal spray composite but are not alloyedwith each other or with the base constituent prior to use in the thermalspray process. The additional constituent aluminum should be present inan amount of about 1 to 15 percent and the yttrium oxide from about 0.5percent and preferably about 1 to 10 percent by weight of the total ofthe base constituent and the additional constituents. The molybdenumand/or cobalt, if either is present as a third or fourth additionalconstituent, may each be present in an amount of about 1 to 10 percentby weight of the total of the alloy and the additional constituents.

The base constituent may contain not only nickel, iron, cobalt, chromiumor combinations of these, and aluminum, as set forth, but mayadditionally contain some amounts of other metallic elements such asyttrium or rare earth metals, and also may contain zirconium, hafnium,titanium refractory metals or metalloids such as silicon, carbon andboron. For example, the base constituent alloy may be of the typesdisclosed in the previously referenced U.S. Pat. No. 4,313,760 and U.K.Patent Application No. GB 2,028,378A. Preferably, however, the baseconstituent alloy is a common, simple alloy such as nickel with 20weight percent chromium, nickel with 16 weight percent chromium and 8weight percent iron, iron with 30 weight percent chromium, or iron with50 weight percent aluminum.

The additional constituents aluminum, yttrium oxide and, optionally,molybdenum and/or cobalt may each contain small amounts of alloyingelements, but preferably each is in substantially pure form; forexample, they should contain less than 5 and preferably less than about2 percent impurities.

The term "composite" as used herein is intended to designate astructurally integral unit and does not include a mere mixture ofconstituents which may be physically separated without any destructionof the structure. Thus, in the case of powder, the term "composite" doesnot include a simple mixture of individual granules of the separate baseconstituent and the additional constituents aluminum, yttrium oxide and,optionally, molybdenum and/or cobalt, but requires that each of theindividual composite granules contain the separate constituents.

When the composite is in the form of a powder, preferably the baseconstituent is in the form of a core particle, and the additionalconstituents are in the form of fine particles such as -10 micron size,desirably secured to the core particle with a binder, preferably anorganic binder. A thermal spray powder of the present invention shouldhave a size generally in the range of about -100 mesh (U.S. StandardMesh Size) to +5 microns, preferably about -120 to +325 mesh. In themore preferable form of powder, the starting core size is equal to, orabout one or two screen sizes smaller than the desired size of the finalpowder, for example, -120 or -140 or -170 mesh size core where the finalpowder is to be -120 +325 mesh. Thus one embodiment contemplated is, forexample, a -120 +325 mesh thermal spray powder having a core of nickel,iron or cobalt alloyed with chromium or aluminum, and additionalparticles of -10 micron aluminum, yttrium oxide and molybdenum and/orcobalt secured to the core with a binder.

The binder material may be any known or conventional binding materialwhich may be used for forming a coating or binding particles together orto a surface. The binder is preferably organic and may be a varnishcontaining a resin as the varnish solids, or may contain a resin whichdoes not depend on solvent evaporation in order to form a cured or setfilm. The binder thus may contain a catalyzed resin as the varnishsolids. Examples of binders which may be used include the conventionalphenolic, epoxy or alkyd varnishes, varnishes containing drying oils,such as tung oil and linseed oil, rubber and latex binders and the like.The binder may alternatively be of the water-soluble type, as forexample, of the polyvinylpyrrolidone or polyvinylalcohol type. Inaddition to organic binders, inorganic binders may be used, such assodium silicate, boric acid, borax, magnesium or other solublecarbonates, nitrates, oxalates or oxychlorides, or colloidal suspensionscontaining oxides.

The coating of the core material with the binder containing theparticles may be effected in any known or desired manner. It is simplynecessary to mix the powdered ingredients together while allowing thebinder to set and dry, which will result in a fairly free-flowing powderconsisting of the core coated with the cladding of the aluminum, yttriumoxide and, optionally, molybdenum and/or cobalt.

In yet another form the core may itself be a composite of fine particlesof the metals, nickel, iron, cobalt and/or chromium. For example, thecore may be a composite of -10 micron nickel particles and 20% of -10micron chromium particles, with -10 micron particles aluminum andyttrium oxide secured to the composite core. The core or the compositepowder itself in accordance with the invention may be manufactured inany known or desired manner, for example, it may be produced with aspray drier as in U.S. Pat. No. 3,617,358. However, the base constituentis preferably a structurally integral component so, for example, thethermal spray composite of the present invention desirably is not formedmerely of fine particles of all ingredients including unalloyed chromiumintermixed together with a binder.

The powders are sprayed in the conventional manner, using a powder-typethermal spray gun, though it is also possible to combine the same intothe form of a composite wire or rod, using plastic or a similar binding,as for example, polyethylene or polyurethane, which decomposes in theheating zone of the gun. In the case of composite wire, the individualconstituents must be incorporated in a single wire. In either wire orpowder composite the constituents must be in intimate contact with eachother.

The composite may be in the form of a wire having a coating sheath ofone material and a core of the others, alternate coating sheaths of twoof the components and a core of the third or a fourth material, a wireformed by twisting or rolling separate wire strands of the components, awire consisting of a sheath of one component and a core containing theother components in powder or compacted form, a wire as described inpreviously referenced U.S. Pat. No. 3,322,515, consisting of a sheath ofone component and a core containing a compacted powder mixture of thissame component material and other components, a wire formed by bending ametal strip around powder fill, a wire consisting of a plastic sheathand a core containing a compacted powder mixture of components, or otherconvenient form. The composite wires should have conventional sizes andaccuracy tolerances for flame spray wires and thus, for example, mayvary in size between 6.4 mm and 20 gauge.

In order for the wires to be satisfactory for spraying with a combustionflame, the same should not cavitate at the tip when heated, and shouldpreferably be capable of forming a pointed or slightly tapered tip whenbeing melted and sprayed. Thus, if the wires have an outer layer orsheath of one component and an inner core of another component, theinner core should not have a lower melting point than the outer sheath,as otherwise the inner core will initially melt, causing cavitation atthe tip. For example, if the wire is in the form of a core with acoating sheath, the coating sheath should be aluminum, as otherwiseduring the spraying operation the wire will initially melt out, causingthe cavitation which will interfere with a satisfactory sprayingoperation. The wire having the melting-point characteristics so as toallow the melting off of the tip without this cavitation is referred toherein as "noncavitating wire."

Where the wire is in the form of a compacted powder core with a coatingsheath of aluminum, the additional constituents are contained in thecore and should be in the form of fine particles of aluminum in anamount of about 1 to 15 percent, yttrium oxide in an amount from about0.5 percent and preferably about 1 to 10 percent and, optionally,molybdenum and/or cobalt each in an amount of about 1 to 10 percent byweight of the core. The core also will contain fine particles of thebalance of the base constituent.

A desirable composite wire for arc spraying with a two-wire arc gun ismade in a known manner by forming a metal strip into a U-shaped crosssection. Core powder is filled into the trough of the "U" and the shapeis closed over the powder to a butt or overlap joint. The resulting wireis drawn to desired size. The sheath may be formed of the baseconstituent, perferably in soft form, for example unalloyed nickel,iron, cobalt or combinations of these metals for ase of fabrications.The core then is formed of powder and contains the balance of thedesired base constituent, namely nickel, iron, cobalt, chromium oralloys thereof. As an example chromium may be included as ferrochromepowder. The core powder also includes the additional constituent. Suchwire for arc spraying need not be non-cavitating.

The spraying is in all respects effected in the conventional mannerpreviously utilized for self-bonding thermal spray material, and inparticular nickel-aluminum composites. Due to the self-bondingcharacteristics, special surface preparation other than good cleaning isnot required, though, of course, conventional surface preparation suchas grit blasting should be utilized to maximize tenacity of the coatingto the substrate surface.

The thermal spray composite in accordance with the invention, ascontrasted with prior known thermal spray materials, produces coatingsthat are not only self-bonding but also are highly resistant to hightemperature oxidation and to oxidizing and reducing atmospherescontaining molten or vaporized contaminants containing sodium, sulfurand chlorine such as are in fuels combusting in a marine environment.Additionally, and very surprisingly, the coatings are highly tenacious,typically having tensile bond strengths substantially greater than thoseof similar prior art composite coatings. Ductility is very good, asevidenced by lack of spalling in sharp angle bending. Coatings also havegood machinability, rendering them useful for one-coat machine-elementapplications. The coatings produced from powders having nickel chromiumalloy cores are especially resistant to oxidizing conditions with orwithout the sodium, sulfur and chlorine. The iron base powders areexcellent in reducing conditions containing contaminants such as sodiumsulfide and sodium chloride.

The powders containing molybdenum and/or cobalt have substantially thesame and in some instances further improvements in corrosion resistanceand tenacity, and additionally have an enhanced resistance to wear andimpact. Cobalt as an additional constituent further improves tenacityand ductility of the coatings as evidenced, for example, by asubstantial increase in allowable thickness of a coating before liftingfrom the subtrate.

Typical applications are energy conversion devices; automotive anddiesel combustion and turbine engines; aircraft and marine turbines;coal, oil, and fossil fueled boilers and power generation systems andcomponents; bond coats for ceramic and metal coatings; and pulp andpaper mill applications.

The following examples are given by way of illustration and notlimitation.

EXAMPLE 1

41 parts by weight of fine aluminum powder of average size about 3.5 to5.5 microns was blended with 9 parts by weight of fine yttrium oxide (Y₂O₃) powder of average size 2.5 microns. A polyvinylpyrolidone (PVP)binder solution containing 100 parts by volume of PVP, 100 parts ofacetic acid and 700 parts of water was prepared separately.Approximately 40 cc of this solution was added to 400 gms of nickel 20percent chromium alloy core which had a mesh size of -140 +325. This wasmixed well. To this, about 50 gms of the blended aluminum and yttriumoxide mixture was slowly added and mixed thoroughly, and the mixingcontinued until the binder dried, leaving a fairly free-flowing powderin which all of the alloy core particles were clad with a dry film whichcontained the aluminum and yttrium oxide particles. The powder waswarmed to about 250° F. to ensure complete drying. The powder was thenscreened and handmilled to reduce the same to a -120 +325 mesh powder.The powder so formed comprised particles of nickel-chromium alloy corewith about 9 percent by weight of fine aluminum particles and 2 percentyttrium oxide particles secured to the core with the binder. The powderwas thermal sprayed on a mild steel plate which had been surface cleanedby smooth grinding. The spraying was effected with a standard plasmaflame gun of the general type described in U.S. Pat. No. 3,145,287 andsold by METCO Inc., Westbury, N.Y. under the trademark METCO Type 7MB,using a GH nozzle with No. 2 powder port, and a powder feeder of thetype described in U.S. Pat. No. 3,501,097 and sold under the trademarkMETCO Type 3MP. Parameters were argon as primary plasma gas at 7.0 bars(100 psi) pressure and 2.3 m³ /hr flow, hydrogen secondary plasma gas at7.0 bars pressure and 0.6 m³ /hr flow; 500 amperes, 70 volts, carriergas 0.42 m³ /hr, powder feed rate 3.6 kg/hr, spray distance 14 cm.Excellent, well-bonded coatings were obtained. Coating hardness averagesRb 85.

EXAMPLE 2

The process of Example 1 is repeated except using as a core an alloy ofiron and 30 percent by weight chromium in place of the nickel-chromium,and spray parameters were as follows: 707 nozzle, No. 6 powder port,nitrogen plasma gas at 3.5 bars (50 psi) pressure and 2.1 m³ /hr flow,600 amperes, 65 volts, spray distance 10 to 15 cm. Similar results areobtained, with coating hardness about Rb 100.

EXAMPLE 3

The process of Example 1 is repeated except fine molybdenum powder of -5microns (average about 2.5 microns) is included in the initial blend ofaluminum and yttrium oxide in an amount equal to 3 parts weight. Theresulting powder comprises a core of nickel chromium alloy having about9 percent by weight aluminum particles, 2 percent by weight yttriumoxide particles and 3 percent by weight molybdenum particles securedthereto. Results are similar.

EXAMPLE 4

The process of Example 2 is repeated except with the addition of thefine molybdenum of Example 3. The resulting powder comprises a core ofiron chromium alloy having about 9 percent by weight aluminum particles,2 percent by weight yttrium oxide particles and 3 percent by weightmolybdenum particles secured thereto. Results are again similar.

EXAMPLE 5

The process of Example 1 is repeated except using a core of iron alloycontaining 25 percent by weight chromium, 21 percent aluminum andpercent yttrium. Also the amounts of individual constituents of aluminumand yttrium oxide are adjusted to produce a powder having 8 percentaluminum and 2 percent yttrium oxide secured to the core with thebinder. Coatings are sprayed as in Example 1. High quality, well-bondedcoatings are obtained.

EXAMPLE 6

The powder made as described in Example 1 is thermal sprayed with thecombustion-type powder spray gun as described in U.S. Pat. No. 2,961,335and sold by METCO Inc., Westbury, N.Y., under the trademark METCO Type5P Thermospray gun. Spraying is effected with a P7G nozzle at a sprayrate of 2.3 kg/hr, using acetylene as fuel at 1.0 bars (15 psi) and 0.96m³ /hr, oxygen at 2.1 bars (30 psi) and 0.96 m³ /hr, and spray distanceof 18 cm. Excellent, well-bonded coatings are obtained.

EXAMPLE 7

The process of Example 1 is repeated except the amounts of individualconstituents of aluminum and yttrium oxide were adjusted to produce apowder having 6 percent by weight aluminum and 2 percent yttrium oxidesecured to the core with the binder. Another powder had 7.5 percentaluminum and 2 percent yttrium oxide secured to the core. Results arevery similar to those of Example 1.

EXAMPLE 8

The process of Example 2 is repeated except with the amount of aluminumadjusted to form a powder having an iron aluminum alloy core and about 5percent by weight aluminum and 2 percent yttrium oxide secured to thecore. Excellent, well-bonded coatings are obtained.

EXAMPLE 9

A powder mixture containing 80 percent by weight of nickel, 20 percentchromium alloy having a particle size between about -400 mesh and +5microns, 5 percent by weight of aluminum of average size about 3.5 to5.5 microns, and 2 percent by weight of yttrium oxide of average sizeless than 5 microns, is thoroughly blended and pressed together in theform of cylindrical briquettes, using a die pressure of 15 bars. Thecylindrical briquettes formed have a diameter of 1.05 cm and a length of1.8 cm. The briquettes are loaded into a drawn aluminum tube of 4 mlength, having a 1.07 cm inner diameter and a 0.104 cm wall thickness.The ends of the tube are plugged closed and the tube then swaged to afinal diameter of 3.175 mm, the surface being maintained free of dents,gouges, scratches and other marks. The wire is then annealed at atemperature between 190° and 390° C. The wire is then coiled andsprayed, using a conventional wire-type flame spray gun sold by METCOInc., of Westbury, N.Y., as the METCO Type 12E wire flame spray gun.Spraying is effected using acetylene at a pressure of 2.7 bar, and airas a blast gas at a pressure of 3.3 bar. The oxygen gas flow ismaintained at 1.46 m³ /hr and the acetylene gas flow at 1.17 m³ /hr. Thewire is sprayed with a spray rate of 13 kg/hr at a spraying distancebetween 10 and 13 cm, with the spray material being deposited on thesurface of a ground and machine-finished cold rolled steel. The sprayedcoating is built up to a thickness of 0.75 mm. The coating is wellbonded and has excellent resistance to hot corrosive conditions.

EXAMPLE 10

The process of Example 1 is repeated except fine cobalt powder of -5microns (average about 2.5 microns) is included in the initial blend ofaluminum and yttrium oxide in an amount equal to 4.5 parts by weight.The resulting powder comprises a core of nickel chromium alloy havingabout 9 percent by weight of aluminum particles, 2 percent by weightyttrium oxide particles and 4.5 percent by weight cobalt particlessecured thereto. An alternative composition has about 8 percent byweight aluminum, 1 percent by weight yttrium oxide and 4 percent byweight cobalt. Excellent, strongly bonded coatings are obtained.

EXAMPLE 11

The process of Example 2 is repeated except fine cobalt powder of -5microns (average about 2.5 microns) is included in the initial blend ofaluminum and yttrium oxide in an amount equal to 4.5 parts by weight.The resulting powder comprises a core of iron chromium alloy havingabout 9 percent by weight of aluminum particles, 2 percent by weight ofyttrium oxide particles, and 4.5 percent by weight of cobalt particlessecured thereto. Excellent, strongly bonded coatings are obtained.

EXAMPLE 12

Composite wires were fabricated by taking a continuous strip of iron andforming it into a U-shape. Core powder blended of 82 parts by weight of-80 +325 mesh ferrochrome (72% chromium, balance iron), 15 parts byweight of -170 +325 mesh aluminum powder and 3 parts by weight of 5 to 7micron (average size) yttrium oxide powder was filled into the trough ofthe strip. This filled U-shape was then closed to an overlap joint andthe wire was drawn to 11 gauge size (2.3 mm; i.e. 0.091 inches). Theiron strip was 12.4 mm (0.489 inches) wide and 0.28 mm (0.011 inches)thick. The iron sheath formed 61 percent by weight of the final wire,which had the composition 70 percent iron, 23 percent chromium, 6%aluminum and 1 percent yttrium oxide. Two such wires were fedsimultaneously through an arc gun sold by METCO Inc. under the trademarkMETCO Type 2RG. Parameters were 200 amperes, 28 volts, atomizing air 2.7bar (40 psi), spray distance 18 cm (7 inches) and spray rate 6.4 kg/hr(14 pounds/hr.) excellent, well bonded coatings were obtained.

Tests were carried out on selected coatings of the examples and knownpowders.

Tensile bond tests on mild steel prepared by grinding or by rough gritblasting were done in accordance with ASTM Standard Model C633-69.Results are given in Table 1 for selected powders of these examples aswell as for several known composite powders. The prior art compositesreported in this and subsequent tables are in the form of alloy coreshaving specified constituents secured thereto with an organic binder.

                  TABLE 1                                                         ______________________________________                                        Tensile Bond Strength (average or typical)                                                    Bond Strength (psi)                                                                         Grit Blast                                      Material Thermal Sprayed                                                                        Ground Surface                                                                            Surface                                         ______________________________________                                        NiCr--9Al--2Y.sub.2 O.sub.3 (Ex. 1)                                                             8025        12,600                                          FeCr--9Al--2Y.sub.2 O.sub.3 (Ex. 2)                                                             7100        10,700                                          NiCr--6Al--2Y.sub.2 O.sub.3 (Ex. 7)                                                             8100        12,200                                          NiCr--9Al (Composite)                                                                           4300        8,000                                           NiCr--6Al (Composite)                                                                           5100        8,100                                           NiCr (Alloy)      4200        7,700                                           Ni--22Cr--10Al--1.0Y (Alloy)                                                                    Does not bond                                                                             6,400                                           FeCr--6Al--3Mo (Composite)                                                                      Does not bond                                                                             7,900                                           Fe--23Cr--6Al--lY.sub.2 O.sub.3                                                                 9700        --                                              (Wire, Ex. 12)                                                                ______________________________________                                    

Oxidation resistance was determined as a percent weight gain measured onsubstrate-free coatings kept at 1100° C. in a static air environment ina high temperature furnace after 30 hours of exposure. Results forvarious coatings including prior known composites and alloys are givenin Table 2.

                  TABLE 2                                                         ______________________________________                                        Oxidation Resistance                                                          Material Thermal Sprayed                                                                         Percent Weight Gain                                        ______________________________________                                        Ni--22Cr--10Al--1.0Y (Alloy)                                                                     1.7                                                        NiCr--9Al--2Y.sub.2 O.sub.3 (Ex. 1)                                                              1.8                                                        NiCr--6Al--2Y.sub.2 O.sub.3 (Ex. 7)                                                              2.2                                                        NiCr--6Al--3Mo (Composite)                                                                       3.1                                                        NiCr--6Al (Composite)                                                                            3.4                                                        NiCr (Alloy)       5.2                                                        (Ni/16Cr/8Fe)--7Al--5Mo                                                                          6.0                                                        (Composite)                                                                   Ni--4.5Al (Composite)                                                                            7.1                                                        ______________________________________                                    

High temperature sulfidation in oxidizing atmosphere were done withmolten salts, using industrial accepted procedures. A cold rolled steelpin of about 0.4 cm diameter and rounded ends was fully coated with theexperimental coating. It was placed half immersed in a cruciblecontaining a salt mixture of 90 percent by weight Na₂ SO₄ and 10 percentNaCl. The crucible with the salt mixture and pin was kept in a hightemperature furnace maintained at 750° C. in static air environment. Atthis temperature the salt melts, and one half of the pin was exposed tomolten salt and the other half of the coated pin was exposed tocorrosive vapor containing sodium, sulfur, chlorine and oxygen species.The test was run for varying lengths of time and stopped if visualdegradation was observed. At the end of the test, the pins were removed,sectioned at both ends and examined metallographically. The results aregiven in Table 3.

                  TABLE 3                                                         ______________________________________                                        Sulfidation - Oxidation                                                                        Time to                                                                       Degrade                                                      Material Thermal Sprayed                                                                       (Hours)  Mode of Attack                                      ______________________________________                                        NiCr--9Al--2Y.sub.2 O.sub.3 (Ex. 1)                                                            55       No Attack                                           FeCr--9Al--2Y.sub.2 O.sub.3 (Ex. 2)                                                            55       No Attack                                           NiCr--6Al--2Y.sub.2 O.sub.3 (Ex. 7)                                                            55       No Attack                                           NiCr--6Al--3Mo (Composite)                                                                     40       Attack - Liquid Phase                               NiCr--6Al (Composite)                                                                          16       Attack - Liquid Phase                               NiCr (Alloy)     16       Attack - Liquid Phase                               (Ni/16Cr/8Fe)--7Al--5Mo                                                                         8       Attack - Liquid and                                 (Composite)               Vapor Phase                                         Ni--4.5Al (Composite)                                                                           8       Attack - Liquid and                                                           Vapor Phase                                         ______________________________________                                    

A standard industrial dry char test was used to simulate boilercorrosion conditions, which comprise a reducing atmosphere with certaincontaminants. In this test, a fused solid was made by fusing togetherfrom 72-73 percent by weight Na₂ Co₃, 17-18 percent Na₂ S and 10 percentof NaCl. This was crushed to a powder, and the coated steel pins werekept embedded in this powder in a dry condition at 455° C. for 2 weeks(336 hours). The pins in this condition were exposed to vapors ofsodium, sulfur and chlorine compounds. At the end of the test, the pinswere removed, lightly cleaned for scale removal, and weight lossdetermined. Also, they were sectioned and evaluated metallographically.Results are presented in Table 4. The wire of Example 12 was alsotested, under slightly different conditions, with excellent results.

                  TABLE 4                                                         ______________________________________                                        Dry Char Test                                                                 Material Thermal Sprayed                                                                        Weight Loss (Milligrams)                                    ______________________________________                                        NiCr--9Al--2Y.sub.2 O.sub.3 (Ex 1)                                                               14                                                         NiCr--6Al (Composite)                                                                            45                                                         (Ni/16Cr/8Fe)--7Al--5Mo                                                                         120                                                         (Composite)                                                                   Ni--4.5Al (Composite)                                                                           140                                                         ______________________________________                                    

Thickness limitation tests were done on mild steel substrates(2.5×7.6×1.3 cm thick) prepared by surface grinding. Coatings wereplasma sprayed up to thicknesses until visible signs of lifting werenoticed. Results are given in Table 5 for selected powders of theseexamples as well as for several known composite powders.

                  TABLE 5                                                         ______________________________________                                        Thickness Limitation                                                          Material Thermal Sprayed                                                                          Thickness (mm)                                            ______________________________________                                        NiCr--9Al--4.5Co--2Y.sub.2 O.sub.3                                                                More than 2.5                                             (Ex. 10)                                                                      NiCr--9Al--2Y.sub.2 O.sub.3 (Ex.1)                                                                More than 1.5                                             NiCr--6Al (Composite)                                                                             1.0                                                       NiCr (Alloy)        Less than 0.5                                             Ni--22Cr--10Al--1.0Y (Alloy)                                                                      Less than 0.5                                             Fe--23Cr--6Al--1Y.sub.2 O.sub.3                                                                   More than 0.5                                             (Wire, Ex. 12)                                                                ______________________________________                                    

While the invention has been described above in detail with reference tospecific embodiments, various changes and modifications which fallwithin the spirit of the invention and scope of the appended claims willbecome apparent to those skilled in the art. The invention is thereforeonly intended to be limited by the appended claims or their equivalents.

What is claimed is:
 1. A thermal spray composite characterized byability to produce tenacious and corrosion resistant coatings,comprising:an alloy base constituent comprising at least one metalselected from the group consisting of nickel, iron, cobalt and chromium;and additional constituents comprising aluminum and yttrium oxide. 2.The thermal spray composite of claim 1 wherein the alloy baseconstituent further comprises aluminum.
 3. The thermal spray compositeof claim 2 wherein the alloy base constituent further comprises at leastone additional element selected from the group consisting of yttrium andrare earth metals.
 4. The thermal spray composite of claim 1 wherein theadditional constituent aluminum is present in amount of about 1 to 15percent by weight and the additional constituent yttrium oxide ispresent in an amount of about 0.5 to 10 percent by weight of the totalof the base constituent and the additional constituents.
 5. The thermalspray composite of claim 1 wherein the additional constituents furthercomprise one or more elements selected fom the group consisting ofmolybdenum and cobalt.
 6. The thermal spray composite of claim 4 whereinthe additional constituents further comprise one or more elementsselected from the group consisting of molybdenum and cobalt each in anamount of about 1 to 10 percent by weight of the total of the baseconstituent and the additional constituents.
 7. The thermal spraycomposite of claim 1 wherein the composite is a powder between about-100 mesh and +5 microns.
 8. The thermal spray composite powder of claim7 wherein the base constituent is in the form of a core, and theadditional constituents are each in the form of -10 micron particlessecured to the core with a binder.
 9. The thermal spray composite ofclaim 1 wherein the base constituent comprises chromium in an amount ofabout 1 to 55 percent by weight of the base constituent, the additionalconstituent aluminum is present in an amount of about 1 to 15 percentand the additional constituent yttrium oxide is present in an amount ofabout 0.5 to 10 percent by weight of the total of the base constituentand the additional constituents.
 10. The thermal spray composite ofclaim 1 wherein the base constituent comprises aluminum in an amount ofabout 1 to 55 percent by weight of the base constituent, the additionalconstituent aluminum is present in an amount of about 1 to 15 percentand the additional constituent yttrium oxide is present in an amount ofabout 0.5 to 10 percent by weight of the total of the base constituentand the additional constituents.
 11. The thermal spray composite ofclaim 1 wherein the base constituent comprises chromium and aluminum ina total amount of about 1 to 55 percent by weight of the baseconstituent, the additional constituent aluminum is present in an amountof about 1 to 15 percent and the additional constituent yttrium oxide ispresent in an amount of about 0.5 to 10 percent by weight of the totalof the base constituent and the additional constituents.
 12. The thermalspray composite of claim 9 or 10 or 11 wherein the additionalconstituents further comprise one or more elements selected from thegroup consisting of molybdenum and cobalt each in an amount of about 1to 10 percent by weight of the total of the base constituents and theadditional constituents.
 13. A thermal spray composite powder betweenabout -100 mesh and +5 microns characterized by ability to producetenacious and corrosion resistant coatings, comprising:a core of analloy comprising nickel and chromium, the chromium being present in anamount of about 1 to 55 percent by weight of the alloy core; andadditional constituents comprising -10 micron aluminum and yttrium oxideparticles secured to the alloy core with an organic binder, the aluminumparticles being present in an amount of about 1 to 15 percent and theyttrium oxide particles being present in an amount of about 0.5 to 10percent by weight of the total of the alloy core and the additionalconstituents.
 14. A thermal spray composite powder of about -100 mesh to+5 microns characterized by ability to produce tenacious and corrosionresistant coatings, comprising:a core of an alloy comprising iron andchromium, the chromium being present in an amount of about 1 to 55percent by weight of the alloy core; and additional constituentscomprising -10 micron aluminum and yttrium oxide particles secured tothe alloy core with an organic binder, the aluminum particles beingpresent in an amount of about 1 to 15 percent and the yttrium oxideparticles being present in an amount of about 0.5 to 10 percent byweight of the total of the alloy core and the additional constituents.15. A thermal spray composite powder of about -100 mesh to +5 micronscharacterized by ability to produce tenacious and corrosion resistantcoatings, comprising:a core of an alloy comprising iron and aluminum,the aluminum being present in an amount of about 1 to 55 percent byweight of the alloy core; and additional constituents comprising -10micron aluminum and yttrium oxide particles secured to the alloy corewith an organic binder, the aluminum particles each being present in anamount of about 1 to 15 percent and the yttrium oxide particles beingpresent in an amount of about 0.5 to 10 percent by weight of the totalof the alloy core and the additional constituents.
 16. The thermal spraycomposite powder of claim 13 or 14 or 15 wherein the additionalconstituents further comprise -10 micron molybdenum particles which aresecured to the alloy core with the organic binder and are present in anamount of about 1 to 10 percent by weight of the total of the alloy coreand the additional constituents.
 17. The thermal spray composite powderof claim 13 or 14 or 15 wherein the additional constituents furthercomprise -10 micron cobalt particles which are secured to the alloy corewith the organic binder and are present in an amount of about 1 to 10percent by weight of the total of the alloy core and the additionalconstituents.
 18. The thermal spray composite powder of claim 15 whereinthe aluminum present in the alloy core is in an amount of about 20 to 55percent by weight.
 19. The thermal spray composite of claim 1 whereinthe composite is in the form of a non-cavitating thermal spray wire. 20.A thermal spray wire characterized by ability to produce tenacious andhot corrosion resistant coatings, comprising:a sheath of aluminum; and acompacted powder core containing a base constituent comprising at leastone metal selected from the group consisting of nickel, iron, cobalt andchromium, the core further comprising aluminum particles and yttriumoxide particles.
 21. The thermal spray wire of claim 20 wherein the baseconstituent is an alloy further comprising aluminum.
 22. The thermalspray wire of claim 20 wherein the aluminum particles are present in anamount of about 1 to 15 percent and the yttrium oxide is present in anamount of about 1 to 10 percent by weight of the core.
 23. The thermalspray wire of claim 22 wherein the core further comprises one or moreelements selected from the group consisting of molybdenum and cobalteach in an amount of about 1 to 10 percent by weight of the core.
 24. Aprocess for producing a tenacious and hot corrosion resistant coatingcomprising thermal spraying a thermal spray composite comprising analloy base constituent comprising at least one metal selected from thegroup consisting of nickel, iron, cobalt and chromium, the compositefurther comprising additional constituents comprising aluminum andyttrium oxide.
 25. The process of claim 24 wherein the base constituentfurther comprises aluminum.
 26. The process of claim 24 wherein thecomposite is a powder between about -100 mesh and +5 microns, the baseconstituent is in the form of a core, and the additional constituentsare each in the form of -10 micron particles secured to the core with abinder.
 27. The process of claim 24 wherein the additional constituentsfurther comprise one or more elements selected from the group consistingof molybdenum and cobalt.
 28. The process of claim 24 wherein thecomposite is a powder between about -100 mesh and +5 microns, the baseconstituent is in the form of a core of an alloy comprising nickel andchromium, the chromium is present in an amount of about 1 to 55 percentby weight of the alloy core, the additional constituents aluminum andyttrium oxide are in the form of -10 micron particles secured to thecore with a binder, the additional constituent aluminum is present in anamount of about 1 to 15 percent and the additional constituent yttriumoxide is present in an amount of about 0.5 to 10 percent by weight ofthe total of the base constituents and the additional constituents. 29.The process of claim 24 wherein the composite is a powder between about-100 and +5 microns, the base constituent is in the form of a core of analloy comprising iron and chromium, the chromium is present in an amountof about 1 to 55 percent by weight of the alloy core, the additionalconstituents aluminum and yttrium oxide are in the form of -10 micronparticles secured to the core with a binder, the additional constituentaluminum is present in an amount of about 1 to 15 percent and theadditional constituent yttrium oxide is present in an amount of about0.5 to 10 percent by weight of the total of the base constituents andthe additional constituents.
 30. The process of claim 24 wherein thecomposite is a powder between about -100 and +5 microns, the baseconstituent is in the form of a core of an alloy comprising iron andaluminum, the aluminum is present in an amount of about 1 to 55 percentby weight of the alloy core, the additional constituents aluminum andyttrium oxide are in the form of -10 micron particles secured to thecore with a binder, the additional constituent aluminum is present in anamount of about 1 to 15 percent and the additional constituent yttriumoxide is present in an amount of about 0.5 to 10 percent by weight ofthe total of the base constituent and the additional constituents. 31.The process of claim 28 or 29 or 30 wherein the additional constituentsfurther comprise -10 micron molybdenum particles which are secured tothe core with the binder and are present in an amount of about 1 to 10percent by weight of the total of the base constituent and theadditional constituents.
 32. The process of claim 28 or 29 or 30 whereinthe additional constituents further comprise -10 micron cobalt particleswhich are secured to the core with the binder and are present in anamount of about 1 to 10 percent by weight of the total of the alloyconstituent and the additional constituents.
 33. A thermal spray wirecharacterized by ability to produce tenacious and hot corrosionresistant coatings, comprising:a sheath formed of a base constituentconsisting essentially of at least one metal selected from the groupconsisting of nickel, iron and cobalt; and a powder core comprisingaluminum particles and yttrium oxide particles.
 34. The thermal spraywire of claim 33 wherein the powder core further comprises particles ofa second base constituent comprising at least one metal selected fromthe group consisting of nickel, iron, cobalt and chromium.
 35. Thethermal spray wire of claim 34 wherein the second base constituentcomprises an alloy of iron and chromium.
 36. The thermal spray wire ofclaim 33 wherein the aluminum particles are present in an amount ofabout 1 to 15 percent and the yttrium oxide is present in an amount ofabout 1 to 10 percent by weight of the core.
 37. The thermal spray wireof claim 35 wherein the core further comprises one or more elementsselected from the group consisting of molybdenum and cobalt each in anamount of about 1 to 10 percent by weight of the core.
 38. A process forproducing a tenacious and hot corrosion resistant coating comprising arcspraying a thermal spray composite comprising:a sheath formed of a baseconstituent consisting essentially of at least one metal selected fromthe group consisting of nickel, iron and cobalt; and a powder corecomprising aluminum particles and yttrium oxide particles.